L-gamma-glutamyl-L-epsilon-lysine and L-glutamyl-L-epsilon-lysine coordination properties towards copper(II) and nickel(II) in aqueous solution

The complex formation between copper(II) or nickel(II) and the alifatic dipeptides L-gamma-glutamyl-L-epsilon-lysine and L-glutamyl-L-epsilon-lysine has been studied in aqueous solution at 25 degrees C and ionic strength 0.1 mol . dm(-3) by means of pH-metric and spectroscopic measurements. In L-gamma-glutamyl-L-epsilon-lysine, two alpha-aminoacidic residues are located at a long distance from each other, and are so far from peptide nitrogen atom that the involvement of dissociated peptide group in coordination is not favored. As a consequence, a small number of complexes is formed, either with copper(II) or nickel(II), and the ML degrees species predominates over the others. In L-glutamyl-L-epsilon-lysine, the amino group of glutamyl residue is adjacent to the peptide group whose dissociation occurs in the presence of both copper(II) and nickel(II). Either potentiometric or spectroscopic data suggest the formation of a high number of complexes with the absence of a dominant species both for copper(II) and nickel(II): MLH(2)(2+), MLH(+), M(2)L(2)H(+), M(2)L(2) degrees, M(2)L(2)H(-1)(-), M(2)L(2)H(-2)(2-), ML(2)H(2) degrees, ML(2)H(-), ML(2)(2-), M(2)L(2+), and M(2)LH(-1)(+) (M = copper(II) or nickel(II)). Since both of the ligands can form homobinuclear complexes with each of the metal ions investigated, mixtures containing both metal ions and one of the peptide ligands have been studied too, but the relevance of heterobinuclear species is not remarkable. The spectroscopic data obtained for single species in solution allowed us to propose the probable structure in solution for several copper(II) complexes.